The problem to which the invention is directed will be explained based on signal transmission between a sensor unit and a superordinated unit, e.g. a measurement transmitter. The field of application can, however, be expanded to general systems, in the case of which signals, especially also power, is transmitted and amplified.
Usually connected to a measurement transmitter is a cable leading to a sensor-containing element, e.g. a sensor, in general, a peripheral device. The connection to a sensor element occurs frequently via a plugged connection, for example, through a galvanically decoupled, especially an inductive, interface. Thus, electrical signals can be transmitted contactlessly. This galvanic isolation provides advantages as regards corrosion protection, voltage isolation, avoiding mechanical wear of a plug, etc.
The inductive interface is usually embodied as a system with two coils, which are, for example, plugged into one another. Typically, the signals applied to the coils transmit both data and energy. The energy must, in such case, be sufficiently large that a sensor-containing element, in general a peripheral device, connected to the plug is supplied sufficiently with energy and, thus, a lasting measurement operation is assured.
For operating the coil connected to the measurement transmitter, sufficient power must be provided. The relevant power range is around 30 mW. In the case of operation of the system from an industry-usual, 4.20 mA electrical current loop with a total-energy budget of e.g. about 30 mW, the efficiency of the system components is a decisive factor.
Usually, digital signals are sent from the measurement transmitter and forwarded via the inductive interface to the peripheral device as an amplitude modulated signal. The amplitude modulation can occur, for example, by generating the coil signals via an oscillatory circuit, which is fed from a variable DC supply voltage. If this DC voltage is increased or decreased as a function of the data signals, then, in the case of suitable design of the oscillatory circuit, the coil voltage of the transmission system can be increased or decreased as a function of the data signals, respectively an amplitude modulation of the coil signals can be implemented.
In a simple implementation, the digital signal is, first of all, converted into an analog signal, which, in the case of a high level (thus a digital “1”), assumes a certain voltage and, in the case of a low level (thus a digital “0”), assumes a voltage around e.g. 10% smaller. This analog signal could be amplified in a linear amplifier, at whose output this voltage is available with a sufficient power, in order therewith lastly to supply the peripheral device with power.
In the case of this type of linear amplifier, a number of integrated circuits are commercially available for the power region of about 20 mW, e.g. a number of operational amplifiers have output transistors capable of providing power of about 30 mW.
The deciding disadvantage in the case of application of a linear amplifier is that those using so-called A-, B- or AB-topologies have poor efficiencies, because the energy available in excess voltage levels is converted in power transistors of the output stage into heat.
For applications, in the case of which high powers are required, e.g. in the case of 200 W audio amplifiers for HIFI applications, where, accordingly also considerable heat generation in the power transistors must be taken into consideration, methods for improving the efficiency are applied. A known method for improving the efficiency is to drive the power transistors no longer in linear operation, but, instead, to utilize switching transistors. This principle is used e.g. in the case of so-called class-D amplifiers.
A class-D amplifier can usually be subdivided into three regions.
The first region is composed of a stage with an input for the wanted signal, which converts the wanted signal into a pulse width modulated signal. Usually this occurs by using a signal generator and a comparator. The comparator compares, first of all, the wanted signal with the comparison signal generated by the signal generator, frequently a triangular signal. The comparator switches its output dependent on which of the two signals has a higher voltage. The result of this so-called pulse width modulation (PWM) is a rectangular signal with different pulse widths and the same frequency as the comparison signal. The pulse widths map the information concerning amplitude and frequency of the audio signal.
In the second region, the PWM signal coming from the comparator is amplified by an amplifier.
The third region is formed by a low-pass filter, which filters out the PWM frequencies.
In the state of the art, in the usual case of application of this technology, the power analog electronics is operated with a power transmission of above 100 mW. The main goal is, in such case, the preventing of power loss and therewith the avoidance of cooling measures. Integrated circuits for the power region above 100 mW are commercially available.
Since, in a power region around 30 mW, cooling measures can be omitted, there is, as a rule, no demand for an improved efficiency, so that energy efficient integrated circuits for this application are not commercially available.